Process for immobilization of glucoamylase

ABSTRACT

Glucoamylase is immobilized with a carbodiimide on a polymer having amino or carboxy functional groups. When the polymer contains amino groups, the glucoamylase is treated with the carbodiimide and then mixed with the polymer or the carbodiimide is added to a mixture of the glucoamylase and the polymer. When the polymer contains carboxy groups, the polymer is treated with the carbodimide and then mixed with the glucomylase. The polymer is produced from acrylic acid, metharylic acid, acrylamide, methacrylamide, N-amino-ethyl-acrylamide and/or N-amino ethyl methacrylamide and a cross-linking agent of the acrylic or allylic type.

This application is a continuation of U.S. application Ser. No. 541,432, filed Oct. 13, 1983, now abandoned.

This invention relates to the preparation of an immobilized glucoamylase enzyme composition, more particularly it is concerned with a process for the immobilization of glucoamylase enzyme on xerogels.

From the organic substances produced in the photosynthesis of plant organisms starch is a highly important industrial raw material. Recently enzymatic transformations of starch and the industrial scale realization of the said procedures have come into prominence. One of the most important transformations of starch is its hydrolytical conversion into glucose which is generally carried out by treating a starch partially hydrolysed by means of an acid or α-amylase with a glucoamylase enzyme. The said glucoamylase enzyme splits off the glucose units from the non-reducing end of the dextrines. When using dissolved glucoamylase only batch-wise or semi-continuous methods are applicable and no economical process is known for the separation of the dissolved enzyme in active form at the end of the reaction. For this reason already at the end of the 1960 years attempts were made for the immobilization of the enzyme by binding the enzyme to diethylaminoethyl cellulose with the aid of 2-amino-4,6-dichloro-s-triazine (Biotechn. Bioeng. 11, 349 (1969)). In the past years other methods have also become known. According to the articles published in Biotechn. Bioeng. 12, 85 (1970), Biotechn. Bioeng. 2273 (1981) and Ann Technol. Agric. 27, 469 (1978) the enzyme can be effectively immobilized through ionic bonds on a DEAE-cellulose, methyl-methacrylate-2-dimethylaminoethyl-methacrylate copolymer or Amberlite IRA 93, respectively, as a carrier. The immobilization on a glass or silica carrier is disclosed in the following prior art reference: Biotech. Bioeng. Symp. 3, 241 (1972); Biotech. Bioeng. 15, 483 (1973); Biotech. Bioeng. 18, 253 (1976); Prikl. Biohim. Mikrobiol. 15, 744 (1979); Enzyme Microb. Technol. 1, 197 (1979); Biotech. Bioeng. 23, 2083 (1981); Enzyme Microb. Technol. 4, 89 (1982) and Enzyme Microb. Technol. 4, 107 (1982), U.S. Pat. No. 3,783,101.

The immobilization on polysaccharide (mainly cellulose) carriers is set forth in Biochim. Biophys. Acta 276, 339 (1972); Biochim. Biophys. Acta 341, 457 (1974); Acta biol. med. germ. 36, 179 (1977); Starch-Starke 30, 414 (1978); Prikl. Biochim. Mikrobiol. 14, 548 (1978) and Collection Czechoslov. Chem. Commun. 45, 2847 (1980) and U.S. Pat. Nos. 3,810,821, 4,063,017, 4,090,022 and 4,048,416. The following references relate to the use of synthetic polymers for the immobilization of glucoamylase: J. Jap. Soc. Starch Sci. 24, 1 (1977); Int. J. Biochem. 8, 501 (1977); Biotech. Bioeng. 20, 1319 (1978) and Enzyme Microb. Technol. 4, 143 (1982) and U.S. Pat. No. 4,066,504.

The known methods comprise the following drawbacks:

use of special, which are not readily available carriers;

use of complicated coupling methods;

lower stability of the immobilized enzyme.

The object of the present invention is to eliminate the disadvantages of the known methods and to provide a process for the preparation of an immobilized glucoamylase composition which produces an enzyme composition suitable for prolonged application and in which the enzyme is bound onto a carrier which is chemically inert, is resistant to the effect of microorganisms, possesses favourable mechanical properties and ensures a large flow velocity. A further object of the invention is to provide a process wherein the enzyme to be immobilized is bound to the carrier under mild chemical and physical conditions.

It has been found that polymers which are produced from acrylic and/or methacrylic acid or acrylamide and/or methacrylamide, or N-aminoethyl-acrylic amide and/or N-aminoethyl-methacrylic amide monomers by means of a cross-linking agent of the acrylic or allylic type (e.g. N,N'-methylene-bis-acrylic amide, ethylene diacrylate or N,N'-diallyl-tartaric amide) and which contain at least 1.0 m.equiv./g--preferably 2-7 m.equiv./g--of --COOH or --NH₂ functional group, fully comply with the above requirements.

When a carrier comprising primary amino groups is used, the carboxy groups of the aspartic or glutamic side chain of the enzyme are converted by treatment with a carbodiimide into a O-acyl-isourea derivative which then reacts with the amino groups of the carrier. When polymeric carriers comprising functional carboxy groups are used, the said carboxy groups are activated by treatment with a carbodiimide being known per se and the thus-treated polymeric carrier is suitable for binding the glucoamylase enzyme. The use of carbodiimides as activator is advantageous because the said activation reaction can be carried out under mild conditions (0°-4° C., pH 6.0-8.0).

According to the present invention there is provided a process for the preparation of an immobilized glucoamylase composition which comprises

(a) treating a glucoamylase enzyme with a carbodiimide compound being soluble in water or soluble in an organic solvent at a temperature below 0° C. and applying the thus-treated enzyme in a solution having a pH value of 4.5-9.0 onto a polymer which is produced from a N-aminoethyl-acrylic amide and/or N-aminoethyl-methacrylic amide, or acrylic amide and/or methacrylic amide monomer by means of a cross-linking agent of the acrylic or allylic type and which comprises at least 1.0 m.equiv./g of --NH₂ groups; or

(b) treating a polymer which is produced from an acrylic acid and/or methacrylic acid or acrylic amide and/or methacrylic amide monomer by means of cross linking agent of the acrylic or allylic type and which comprises at least 1.0 m.equiv./g of functional --COOH groups with a carbodiimide compound soluble in water or soluble in an organic solvent at a temperature below 0° C. and applying onto the thus-treated carrier a glucoamylase enzyme in a solution having a pH value between 4.5 and 9.0.

In both cases (a) and (b) the product obtained is filtered, washed and, if desired, dried.

As the polymer, preferably, an acrylic amide-N,N'-methylene-bis-acrylic amide acrylic acid copolymer can be used which has an exclusion molecular weight limit of 100 000 daltons (Acrylex C-100, X-200, C-300). Alternatively an acrylic amide-N,N'-methylene-bis-acrylic amide-N-aminoethyl-acrylic amide copolymer (Acrylex A) can be used as well.

According to the process of the present invention any glucoamylase enzyme of whatever origin and separated by any known method can be used.

As activating carbodiimide compound e.g. N-cyclohexyl-N'-[2-(4-morpholinyl)-ethyl]-carbodiimide-methyl-p-toluene-sulfonate or N-ethyl-N'-(3-dimethyl-amino-propyl)-carbodiimide-hydrochloride can be used. It is preferred to use a water soluble carbodiimide. From the carbodiimide compounds which are soluble in an organic solvent, those compounds suitable for use in the present invention are only those compounds soluble in the cold organic solvent i.e. which are soluble at a temperature below 0° C.

The glucoamylase or the O-acyl isourea derivative, respectively, can be applied onto the carrier at a pH between 4.5 and 9.0--preferably at 7.5. As reaction medium preferably a 0.1 molar potassium phosphate solution or a 0.1 molar sodium acetate solution can be used (pH 7.0-7.5).

The immobilized enzyme composition formed by the coupling reaction is washed by methods known per se and, if desired, dried. The enzyme compositions of the present invention can be stored at 0°-4° C. in aqueous suspension as well.

The activity of the immobilized enzyme composition amounts to 0.9-8.4 units/g xerogel and this value is very advantageous from the point of view of practical use (1 unit corresponds to an enzyme amount which sets free 1 g of glucose from soluble starch at pH 4.0 and at 60° C.).

The stability of the glucoamylase enzyme immobilized on acrylic amide N,N'-methylene-bis-acrylic amide acrylic acid copolymer (Acrylex C-100) by the process of the present invention is larger than or nearly identical with that of the soluble enzyme in a broad pH interval (see Table 1).

                  TABLE 1                                                          ______________________________________                                         Half-period (t.sub.1/2 of the thermal inactivation of                          glucoamylase enzyme compositions at 60° C.                                        Half-period of inactivation (t.sub.1/2)                                        (minutes)                                                            pH value of                 Immobilized                                        incubation  Dissolved glucoamylase                                                                         glucoamylase                                       ______________________________________                                         3.0         32              80                                                 3.5         124             113                                                4.0         133             280                                                4.5         105             130                                                5.0         44              42                                                 5.5         15              15                                                 ______________________________________                                    

In the presence of a substrate and at the optimal pH value of application the said effect is still more pronounced.

Further details of the present invention are to be found in the following Examples without limiting the scope of protection to the said Examples.

EXAMPLE 1

0.5 g of an Acrylex A type xerogel (binding capacity--i.e. --NH₂ content--2.75 m.equiv./g; particle size 100-320 μm) is suspended in 25 ml of a 0.1 molar potassium phosphate buffer (pH 7.0), whereupon 1.7 ml of a glucoamylase solution (117.7 mg/ml) are added with constant stirring. The reaction mixture is stirred for 10 minutes whereupon 0.5 g of N-cyclohexyl-N'-[2-(4-morpholinyl)-ethyl]-carbodiimide dissolved in 25 ml of a cold (+4° C.) buffer are added. At 0°-4° C. the complete reaction time amounts to 48 hours and during this period the reaction mixture is stirred twice for 6 hours each. The gel is removed by filtration and washed three times with 50 ml of a 0.1 molar potassium phosphate buffer each (pH 7.0), three times with 50 ml of a 0.1 molar potassium phosphate buffer each comprising 1 mole of sodium chloride (pH 7.0) and three times with 50 ml of a 0.1 molar potassium phosphate buffer each (pH 7.0). The gel is finally washed salt-free with distilled water and subjected to lyophilization. Thus 0.6 g of immobilized glucoamylase is obtained. Activity: 1.6 g of glucose are set free/hour/g.

EXAMPLE 2

0.5 g of Acrylex C-100 xerogel (binding capacity--i.e. --COOH content--6.2 m.equiv./g, particle size 100-320 μm) is suspended in 50 ml of a 0.1 molar sodium acetate buffer (pH 4.5) whereupon 1 g of N-cyclohexyl-N'-[2-(4-morpholinyl)-ethyl]-carbodiimide-methyl-p-toluene-sulfonate is added on an icecold water-bath with constant stirring. The reaction mixture is stirred for 10 minutes, whereupon 1.7 ml of a glucoamylase solution (117.7 mg/ml) are added and the pH value of the reaction mixture is adjusted to 4.5. At 0°-4° C. the complete reaction time amounts to 48 hours and during this period the suspension is stirred twice each time for 6 hrs. The pH is controlled at the beginning every half an hour and later once an hour and is, if necessary, re-adjusted. After the reaction has been completed, the gel is removed by filtration and washed three times with 50 ml of a 0.1 molar sodium acetate buffer each (pH 4.5), three times with 50 ml of a 1.0 molar sodium acetate buffer comprising 1.0 mole of sodium chloride each (pH 4.5) and three times with 50 ml of a 0.1 molar sodium acetate buffer each (pH 4.5). The gel is finally washed salt-free with distilled water and subjected to lyophilization. Thus 0.7 g of immobilized glucoamylase is obtained. Activity: 0.85 g of glucose is set free/hour/g xerogel.

EXAMPLE 3

One proceeds according to Example 2 except that the reaction is carried out for 24 hours. Yield: 0.65 g of immobilized glucoamylase. Activity: 0.79 g of glucose is set free/hour/g xerogel.

EXAMPLE 4

One proceeds according to Example 2 except that the reaction is carried out for 72 hours. Yield: 0.63 g of immobilized glucoamylase. Activity: 0.82 g of glucose is set free/hour/g xerogel.

EXAMPLE 5

0.5 g of Acrylex C-100 xerogel (binding capacity--i.eg. --COOH content--6.2 m.equiv./g, particle size 100-320 μm) is suspended in 50 ml of a 0.1 molar sodium acetate buffer (ph 5.0) whereupon 1 g of N-cyclohexyl-N'-[2-(4-morpholinyl)-ethyl]-carbodiimide-methyl-p-toluene-sulfonate is added on an icecold water-bath under constant stirring. The reaction mixture is stirred for 10 minutes, whereupon 1.7 ml of a glucoamylase solution (117.7 mg/ml) are added and the pH value of the reaction mixture is adjusted to 4.5. At 0°-4° C. the complete reaction time amounts to 48 hours and during this period the suspension is stirred twice each time for 6 hours. The pH is controlled at the beginning every half an hour and later once an hour and is, if necessary, re-adjusted. After the reaction has been completed, the gel is removed by filtration and washed three times with 50 ml of a 0.1 molar sodium acetate buffer each (pH 5.0 ), three times with 50 ml of a 1.0 molar sodium acetate buffer comprising 1.0 mole of sodium chloride each (pH 5.0) and three times with 50 ml of a 0.1 molar sodium acetate buffer each (pH 5.0). The gel is finally washed salt-free with distilled water and subjected to lyophilization. Thus 0.7 g of immobilized glucoamylase is obtained. Activity: 0.75 g.

EXAMPLE 6

0.5 g of Acrylex C-100 xerogel (binding capacity--i.e. --COOH content--6.2 m.equiv./g, particle size 100-320 μm) is suspended in 50 ml of a 0.1 molar sodium acetate buffer (pH 6.0) whereupon 1 g of N-cyclohexyl-N'-[2-(4-morpholinyl)-ethyl]-carbodiimide-methyl-p-toluene-sulfonate is added on an icecold water-bath under constant stirring. The reaction mixture is stirred for 10 minutes, whereupon 1.7 ml of a glucoamylase solution (117.7 mg/ml) are added and the pH value of the reaction mixture is adjusted to 6.0. At 0°-4° C. the complete reaction time amounts to 48 hours and during this period the suspension is stirred twice for 6 hours each. The pH is controlled at the beginning every half an hour and later once an hour and is if necessary re-adjusted. After the reaction has been completed, the gel is removed by filtration and washed three times with 50 ml of a 0.1 molar sodium acetate buffer each (pH 6.0), three times with 50 ml of a 1.0 molar sodium acetate buffer comprising 1.0 mole of sodium chloride each (pH 6.0) and three times with 50 ml of a 0.1 molar sodium acetate buffer each (pH 6.0). The gel is finally washed salt-free with distilled water and subjected to lyophilization. Thus 0.7 g of immobilized glucoamylase is obtained. Activity: 1.35 g of glucose are set free/hour/g xerogel.

EXAMPLE 7

0.5 g of Acrylex C-100 xerogel (binding capacity--i.e. --COOH content--6.2 m.equiv./g, particle size 100-320 μm) is suspended in 50 ml of a 0.1 molar sodium acetate buffer (pH 7.0), whereupon 1 g of N-cyclohexyl-N'-[2-(4-morpholinyl)-ethyl]-carbodiimide-methyl-p-toluene-sulfonate is added on an icecold water-bath under constant stirring. The reaction mixture is stirred for 10 minutes, whereupon 1.7 ml of a glucoamylase solution (117.7 mg/ml) are added and the pH value of the reaction mixture is adjusted to 7.0. At 0°-4° C. the complete reaction time amounts to 48 hours and during this period the suspension is stirred twice for 6 hours each. The pH is controlled at the beginning every half an hour and later once an hour and is, if necessary, re-adjusted. The reaction having been completed the gel is removed by filtration and washed three times with 50 ml of a 0.1 molar sodium acetate buffer each (pH 7.0), three times with 50 ml of a 1.0 molar sodium acetate buffer comprising 1.0 mole of sodium chloride each (pH 7.0) and three times with 50 ml of a 0.1 molar sodium acetate buffer each (pH 7.0). The gel is finally washed salt-free with distilled water and subjected to lyophilization. Thus 0.6 g of immobilized glucoamylase is obtained. Activity: 3.0 g. of glucose is set free/hour/g xerogel.

EXAMPLE 8

0.5 g of Acrylex C-100 xerogel (binding capacity--i.e. --COOH content--6.2 m.equiv./g, particle size 100-320 μm) is suspended in 50 ml of a 0.1 molar sodium acetate buffer (pH 7.5), whereupon 1 g of N-cyclohexyl-N'-[2-(4-morpholinyl)-ethyl]-carbodiimide-methyl-p-toluene-sulfonate is added on an icecold water-bath under constant stirring. The reaction mixture is stirred for 10 minutes, whereupon 1.7 ml of a glucoamylase solution (117.7 mg/ml) are added and the pH value of the reaction mixture is adjusted to 7.5. At 0°-4° C. the complete reaction time amounts to 48 hours and during this period the suspension is stirred twice for 6 hours each. The pH is controlled at the beginning every half an hour and later once an hour and is, if necessary, re-adjusted. The reaction having been completed the gel is removed by filtration and washed three times with 50 ml of a 0.1 molar sodium acetate buffer each (pH 7.5), three times with 50 ml of a 1.0 molar sodium acetate buffer comprising 1.0 mole of sodium chloride each (pH 7.5) and three times with 50 ml of a 0.1 molar sodium acetate buffer each (pH 7.5). The gel is finally washed salt-free with distilled water and subjected to lyophilization. Thus 0.75 g of immobilized glucoamylase is obtained. Activity: 5.7 g of glucose is set free/hour/g xerogel.

EXAMPLE 9

0.5 g of Acrylex C-100 xerogel (binding capacity--i.e. --COOH content--6.2 m.equiv./g, particle size 100-320 μm) is suspended in 50 ml of a 0.1 molar sodium acetate buffer (pH 8.0), whereupon 1 g of N-cyclohexyl-N'-[2-(4-morpholinyl)-ethyl]-carbodiimide-methyl-p-toluene-sulfonate is added on an icecold water-bath under constant stirring. The reaction mixture is stirred for 10 minutes, whereupon 1.7 ml of a glucoamylase solution (117.7 mg/ml) are added and the pH value of the reaction mixture is adjusted to 8.0. At 0°-4° C. the complete reaction time amounts to 48 hours and during this period the suspension is stirred twice for 6 hours each. The pH is controlled at the beginning every half an hour and later once an hour and is, if necessary, re-adjusted. The reaction having been completed the gel is removed by filtration and washed three times with 50 ml of a 0.1 molar sodium acetate buffer each (pH 8.0), three times with 50 ml of a 1.0 molar sodium acetate buffer comprising 1.0 mole of sodium chloride each (pH 8.0) and three times with 50 ml of a 0.1 molar sodium acetate buffer each (pH 8.0). The gel is finally washed salt-free with distilled water and subjected to lyophilization. Thus 0.65 g of immobilized glucoamylase is obtained. Activity: 3.9 g of glucose is set free/hour/g xerogel.

EXAMPLE 10

0.5 g of Acrylex C-100 xerogel (binding capacity--i.e. --COOH content--6.2 m.equiv/g, particle size 100-320 μm) is suspended in 50 ml of a 0.1 molar sodium acetate buffer (pH 8.5), whereupon 1 g of N-cyclohexyl-N'-[2-(4-morpholinyl)-ethyl]-carbodiimide-methyl-p-toluene-sulfonate is added on an icecold water-bath under constant stirring. The reaction mixture is stirred for 10 minutes, whereupon 1.7 ml of a glucoamylase solution (117.7 mg/ml) are added and the pH value of the reaction mixture is adjusted to 8.5. At 0°-4° C. the complete reaction time amounts to 48 hours and during this period the suspension is stirred twice for 6 hours each. The pH is controlled at the beginning every half an hour and later once an hour and is, if necessary, re-adjusted. The reaction having been completed the gel is removed by filtration and washed three times with 50 ml of a 0.1 molar sodium acetate buffer each (pH 8.5), three times with 50 ml of a 1.0 molar sodium acetate buffer comprising 1.0 mole of sodium chloride each (pH 8.5) and three times with 50 ml of a 0.1 molar sodium acetate buffer each (pH 8.5). The gel is finally washed salt-free with distilled water and subjected to lyophilization. Thus 0.7 g of immobilized glucoamylase is obtained. Activity: 2.2 g of glucose is set free/hour/g xerogel.

EXAMPLE 11

0.5 g of Acrylex C-100 xerogel (binding capacity--i.e. --COOH content--6.2 m.equiv./g, particle size 100-320 μm) is suspended in 50 ml of a 0.1 molar sodium acetate buffer (pH 9.0), whereupon 1 g of N-cyclohexyl-N'-[2-(4-morpholinyl)-ethyl]-carbodiimide-methyl-p-toluene-sulfonate is added on an icecold water-bath under constant stirring. The reaction mixture is stirred for 10 minutes, whereupon 1.7 ml of a glucoamylase solution (117.7 mg/ml) are added and the pH value of the reaction mixture is adjusted to 9.0. At 0°-4° C. the complete reaction time amounts to 48 hours and during this period the suspension is stirred twice for 6 hours each. The pH is controlled at the beginning every half an hour and later once an hour and is, if necessary, re-adjusted. The reaction having been completed the gel is removed by filtration and washed three times with 50 ml of a 0.1 molar sodium acetate buffer each (pH 9.0), three times with 50 ml of a 1.0 molar sodium acetate buffer comprising 1.0 mole of sodium chloride each (pH 9.0) and three times with 50 ml of a 0.1 molar sodium acetate buffer each (pH 9.0). The gel is finally washed salt-free with distilled water and subjected to lyophilization. Thus 0.8 g of immobilized glucoamylase is obtained. Activity: 1.25 g of glucose are set free/hour/g xerogel.

EXAMPLE 12

0.5 g of Acrylex C-100 xerogel (binding capacity--i.e. --COOH content--6.2 m.equiv./g, particle size 100-320 μm) is suspended in 50 ml of a 0.1 molar potassium phosphate buffer (pH 7.5), whereupon 0.5 g of N-cyclohexyl-N'-[2-(4-morpholinyl)-ethyl]-carbodiimide-methyl-p-toluene sulfonate is added in an icecold water-bath under constant stirring. The reaction mixture is stirred for 10 minutes whereupon 2 ml of a glucoamylase solution (125 mg/ml) are added and the pH of the reaction mixture is adjusted to 7.5. At 0°-4° C. the complete reaction time amounts to 48 hours during this period the suspension is stirred twice for 6 hours each. At the beginning of the reaction the pH is controlled every half an hour and later once an hour and is adjusted, if necessary. The reaction having been completed the gel is separated by filtration and washed three times with 50 ml of a 0.1 molar potassium phosphate buffer each (pH 7.5), three times with 50 ml of a 0.1 molar potassium phosphate buffer comprising 1 mole of sodium chloride each (pH 7.5) and three times with 50 ml of a 0.1 molar potassium phosphate buffer each (pH 7.5). The gel is finally washed salt-free with distilled water and subjected to lyophilization. Thus 0.65 g of immobilized glucoamylase are obtained. Activity: 1.32 g of glucose are set free/hour/g xerogel.

EXAMPLE 13

0.5 g of Acrylex C-100 xerogel (binding capacity--i.e. --COOH content--6.2 m.equiv./g, particle size 100-320 μm) is suspended in 50 ml of a 0.1 molar potassium phosphate buffer (pH 7.5), whereupon 0.5 g of N-cyclohexyl-N'-[2-(4-morpholinyl)-ethyl]-carbodiimide-methyl-p-toluene sulfonate is added in an icecold water-bath under constant stirring. The reaction mixture is stirred for 10 minutes whereupon 1 ml of a glucoamylase solution (125 mg/ml) are added and the pH of the reaction mixture is adjusted to 7.5. At 0°-4° C. the complete reaction time amounts to 48 hours and during this period the suspension is stirred twice for 6 hours each. At the beginning of the reaction the pH is controlled every half an hour and later once an hour and is adjusted, if necessary. The reaction having been completed the gel is separated by filtration and washed three times with 50 ml of a 0.1 molar potassium phosphate buffer each (pH 7.5), three times with 50 ml of a 0.1 molar potassium phosphate buffer comprising 1 mole of sodium chloride each (pH 7.5) and three times with 50 ml of a 0.1 molar potassium phosphate buffer each (pH 7.5). The gel is finally washed salt-free with distilled water and subjected to lyophilization. Thus 0.6 g of immobilized glucoamylase are obtained. Activity: 2.08 g of glucose are set free/hour/g xerogel.

EXAMPLE 14

0.5 g of Acrylex C-100 xerogel (binding capacity--i.e. --COOH content--6.2 m.equiv./g, particle size 100-320 μm) is suspended in 50 ml of a 0.1 molar potassium phosphate buffer (pH 7.5), wherein 1 g of N-cyclohexyl-N'-[2-(4-morpholinyl)-ethyl]-carbodiimide-methly-p-toluene sulfonate is added in an icecold water-bath under constant stirring. The reaction mixture is stirred for 10 minutes whereupon 1.7 ml of a glucoamylase solution (117.7 mg/ml) are added and the pH of the reaction mixture is adjusted to 7.5. At 0°-4° C. the complete reaction time amounts to 48 hours and during this period the suspension is stirred twice for 6 hours each. At the beginning of the reaction the pH is controlled every half an hour and later once an hour and is adjusted, if necessary. The reaction having been completed the gel is separated by filtration and washed three times with 50 ml of a 0.1 molar potassium phosphate buffer each (pH 7.5), three times with 50 ml of a 0.1 molar potassium phosphate buffer comprising 1 mole of sodium chloride each (pH 7.5) and three times with 50 ml of a 0.1 molar potassium phosphate buffer each (pH 7.5). The gel is finally washed salt-free with distilled water and subjected to lyophilization. Thus 0.7 g of immobilized glucoamylase are obtained. Activity: 4.7 g of glucose are set free/hour/g xerogel.

                                      TABLE 2                                      __________________________________________________________________________     Effect of the pH on the immobilization of glucoamelyase enzyme on              Acrylex C-100 carrier                                                                   Immobilized                                                                           Activity recovered                                             pH of the reaction                                                                      activity,                                                                             in dissolved form                                                                        Loss of activity                                                                       Activity of the product                      mixture  (%)    (%)       (%)     /unit/g xerogel                              __________________________________________________________________________     4.5      0.4    56.5      43.1    0.85                                         5.0      0.5    54.8      44.7    0.75                                         6.0      0.7    52.2      47.1    1.35                                         7.0      1.3    49.1      49.6    3.00                                         7.5      4.3    45.3      50.4    5.70                                         8.0      2.5    41.0      56.5    3.90                                         8.5      1.1    44.0      54.9    2.20                                         9.0      0.7    42.0      57.3    1.25                                         __________________________________________________________________________

                                      TABLE 3                                      __________________________________________________________________________     Effect of the composition of the reaction mixture on the immobilization        of glucoamylase enzyme on an Acrylex C-100 carrier                             Composition of the                                                             reaction mixture      Activity recovered                                                                               Activity of the                        .sup.1 AC-100                                                                       .sup.2 CMC                                                                         .sup.3 GA                                                                         Immobilized activity                                                                     in dissolved form                                                                        Loss of activity                                                                       product                                (mg) (mg)                                                                               (mg)                                                                              (%)       (%)       (%)     /unit/g xerogel                        __________________________________________________________________________     500  500 125                                                                               0.6       75.7      24.3    1.32                                   500  500 250                                                                               0.5       56.1      43.4    2.08                                   500  1000                                                                               200                                                                               2.3       45.2      52.5    4.70                                   __________________________________________________________________________      .sup.1 = AC100, Acrylex C100                                                   .sup.2 = CMC,                                                                  N--cyclohexylN'--[2-(4-morpholinyl)-ethyl]-carbodiimide-methyl-p-toluene-     ulfonate                                                                        .sup.3 = GA, glucoamylase.                                               

EXAMPLE 15

0.5 g of Acrylex C-200 xerogel (binding capacity--i.e. --COOH content--7.49 m.equiv/g, particle size 120-320 μm) is suspended in 100 ml of a 0.1 molar potassium phosphate buffer (pH 7.5), whereupon 1.25 g of N-cyclohexyl-N'-[2-(4-morpholinyl)-ethyl]-carbodiimide-methyl-p-toluene sulfonate is added in an icecold water-bath under constant stirring. The reaction mixture is stirred for 10 minutes whereupon 2.6 ml of a glucoamylase solution (120 mg/ml) are added and the pH of the reaction mixture is adjusted to 7.5. At 0°-4° C. the complete reaction time amounts to 48 hours and during this period the suspension is stirred twice for 6 hours each. At the beginning of the reaction the pH is controlled every half an hour and later once an hour and is adjusted, if necessary. The reaction having been completed the gel is separated by filtration and washed three times with 100 ml of a 0.1 molar potassium phosphate buffer each (pH 7.5), three times with 100 ml of a 0.1 molar potassium phosphate buffer comprising 1 mole of sodium chloride each (pH 7.5) and three times with 100 ml of a 0.1 molar potassium phosphate buffer each (pH 7.5). The gel is finally washed salt-free with distilled water and subjected to lyophilization. Thus 0.7 g of immobilized glucoamylase are obtained. Activity: 7.5 g of glucose are set free/hour/g xerogel.

EXAMPLE 16

0.5 g of Acrylex C-300 xerogel (binding capacity--i.e. --COOH content--7.33 m.equiv./g, particle size 40-120 μm) is suspended in 150 ml of a 0.1 molar potassium phosphate buffer (pH 7.5), whereupon 1.2 g of N-cyclohexyl-N'-[2-(4-morpholinyl)-ethyl]-carbodiimide-methyl-p-toluene sulfonate is added in an icecold water-bath under constant stirring. The reaction mixture is stirred for 10 minutes whereupon 2.55 ml of a glucoamylase solution (120 mg/ml) are added and the pH of the reaction mixture is adjusted to 7.5. At 0°-4° C. the complete reaction time amounts to 48 hours and during this period the suspension is stirred twice for 6 hours each. At the beginning of the reaction the pH is controlled every half an hour and later once an hour and is adjusted, if necessary. The reaction having been completed the gel is separated by filtration, and washed three times with 150 ml of a 0.1 molar potassium phosphate buffer each (pH 7.5), three times with 150 ml of a 0.1 molar potassium phosphate buffer comprising 1 mole of sodium chloride each (pH 7.5) and three times with 150 ml of a 0.1 molar potassium phosphate buffer each (pH 7.5). The gel is finally washed salt-free with distilled water and subjected to lyophilization. Thus 0.7 g of immobilized glucoamylase are obtained. Activity: 8.4 g of glucose are set free/hour/g xerogel.

                                      TABLE 4                                      __________________________________________________________________________     Immobilization of glucoamylase enzyme                                                              Activity recovered                                                                               Activity of the                                  Immobilization activity                                                                    in dissolved from                                                                        Loss of activity                                                                       product                                  Carrier (%)         (%)       (%)     /(unit/g xerogel)                        __________________________________________________________________________     Acrylex A                                                                              0.7         39.0      60.3    1.6                                      Acrylex C-100                                                                          4.3         45.3      50.4    5.7                                      Acrylex C-200                                                                          1.6         59.0      39.4    7.5                                      Acrylex C-300                                                                          1.8         69.5      28.7    8.4                                      __________________________________________________________________________ 

What we claim is:
 1. A process for the preparation of an immobilized glucoamylase enzyme composition comprising:treating a glucoamylase enzyme with a carbodiimide compound which is water-soluble or soluble in an organic solvent at a temperature below 0° C.; mixing the treated glucoamylase and a polymer which is suspended and swollen in a buffer solution having a pH of from 4.5 to 9, said polymer being formed by polymerizing a monomer selected from the group consisting of N-aminoethyl-acrylamide, N-aminoethyl-methacrylamide, acrylamide, methacrylamide and mixtures thereof in the presence of a cross-linking agent selected from the group consisting of N,N'-methylene-bis-acrylic amide, ethylene diacrylate and N,N'-diallyl-tartaric amide, which polymers contain at least 1.0 m. equiv./g of --NH₂ functional groups; stirring the mixture at 0° to 4° C. for a period of 24 to 72 hours to produce an immobilized glucoamylase enzyme composition; filtering the immobilized glycoamylase enzyme composition produced; and thereafter washing the product composition several times with a buffer solution having a pH of 4.5 to 9 and then washing the product composition with water.
 2. A process for the preparation of an immobilized glucoamylase enzyme composition comprising:mixing a glucoamylase enzyme and a polymer which is suspended and swollen in a buffer solution having a pH of from 4.5 to 9, said polymer being formed by polymerizing a monomer selected from the group consisting of N-aminoethylacrylamide, N-aminoethyl-methacrylamide, acrylamide, methacrylamide and mixtures thereof in the presence of a cross-linking agent selected from the group consisting of N,N'-methylene-bis-acrylic amide, ethylene diacrylate and N,N'-diallyl-tartaric amide, which polymers contain at least 1.0 m. equiv./g of --NH₂ functional groups; adding a carbodiimide compound which is water-soluble or soluble in an organic solvent at a temperature below 0° C. to the mixture thereby ensuring a controlled activation of the carboxy groups of the glycoamylase; stirring the mixture at 0° to 4° C. for a period of 24 to 72 hours to produce an immobilized glucoamylase enzyme composition; filtering the immobilized glycoamylase enzyme composition produced; and thereafter washing the product composition several times with a buffer solution having a pH of 4.5 to 9 and then washing the product composition with water.
 3. A process for the preparation of an immobilized glucoamylase enzyme composition comprising:treating a polymer which is suspended and swollen in a buffer solution having a pH of from 4.5 to 9, said polymer being formed by polymerizing a monomer selected from the group consisting of acrylic acid, methacrylic acid, acrylamide, methacrylamide and mixtures thereof in the presence of a cross-linking agent selected from the group consisting of N,N'-methylene-bis-acrylic amide, ethylene diacrylate and N,N'-diallyl-tartaric amide, which polymers contain at least 1.0 m. equiv./g of --COOH functional groups, with a carbodiimide compound which is water-soluble or soluble in an organic solvent at a temperature below 0° C. to form an activated carrier; mixing a glycoamylase solution having a pH of 4.5 to 9 with the activated carrier; stirring the mixture at 0° to 4° C. for a period of 24 to 72 hours to produce an immobilized glucoamylase enzyme composition; filtering the immobilized glycoamylase enzyme composition produced; and thereafter washing the product composition several times with a buffer solution having a pH of 4.5 to 9 and then washing the product composition with water.
 4. A process according to claim 1 or 2, wherein the polymer is an acrylamide-N,N'-methylene-bis-acrylamide-N-aminomethyl-acrylamide copolymer which has an exclusion molecular weight limit of at least 100,000 daltons.
 5. A process according to claim 3, wherein the polymer is an acrylamide-N-N'-methylene-bis-acrylamide acrylic acid copolymer which has an exclusion molecular weight limit of at least 100,000 daltons.
 6. A process according to any of claims 1 to 3, wherein the carbodiimide is N-cyclohexyl-N'-[2-(4-morpholinyl)-ethyl]-carbodiimide-methyl-p-toluene-sulfonate or N-ethyl-N'-(3-dimethylaminopropyl)carbodiimide-hydrochloride.
 7. A process according to any of claims 1 to 3 or 5, wherein a 0.1 molar potassium phosphate or sodium acetate solution is used as the buffer solution.
 8. A process according to any of claims 1 to 3, wherein said period is 48 hours.
 9. A process according to any of claims 1 to 3, wherein the product is subjected to lyophilization.
 10. A process according to claim 6, wherein a 0.1 molar potassium phosphate or sodium acetate solution is used as the buffer solution.
 11. A process according to claim 4, wherein a 0.1 molar potassium phosphate or sodium acetate solution is used as the buffer solution. 